Resistive touchscreens and touchpads are known in the prior art, and often find application in touchscreens or touchpads that work in conjunction with a stylus. When the stylus is pressed downwardly against the touchscreen or touchpad, upper and lower resistive electrodes are brought into contact with one another and the location of the stylus is determined by calculating the location where the two arrays have shorted out. Resistive touchscreens typically attenuate light passing therethrough substantially owing to the relatively large amounts of Indium Tin Oxide (“ITO”) required to form the resistive electrodes thereof.
Capacitive touchscreens, such as those found in IPHONEs™ provide two advantages respecting resistive touchscreens. First, they function with almost no pressure being applied by a finger, so they do not present problems associated with stiction and are comfortable to use. This is particularly important for swipe and pinch gestures, where the finger has to slide over a touch surface. Second, some capacitive touchscreens support the measurement of multiple finger locations simultaneously (commonly known as “multi-touch” capability).
The primary technical drawback of a traditional capacitive touchscreen or touchpad is the lack of support for a stylus (in addition to a finger). A stylus provides a more precise pointing device, permits the entry of complicated text and characters, and does not obscure the target as much as a finger. Although capacitive touchscreens have been made to work with a stylus, it is believed this has only been accomplished with an electrically conductive stylus having a tip size comparable to that of a human finger. This, of course, can defeat the benefits arising from using a stylus.
Another important aspect of touchscreens and touchpads has to do with the particular type of technology employed in sensing and measuring changes in capacitance. Two principal capacitive sensing and measurement technologies currently find use in most touchpad and touchscreen devices. The first such technology is that of self-capacitance. By way of example, many devices manufactured by SYNAPTICS™ employ self-capacitance measurement techniques, as do integrated circuit (IC) devices such as the CYPRESS PSOC.™ Self-capacitance involves measuring the self-capacitance of a series of electrode pads using techniques such as those described in U.S. Pat. No. 5,543,588 to Bisset et al. entitled “Touch Pad Driven Handheld Computing Device” dated Aug. 6, 1996.
Self-capacitance is a measure of how much charge has accumulated on an object held at a given voltage (Q=CV). Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external grounded objects are brought close to the electrode, additional charge is attracted to the electrode. As a result, the self-capacitance of the electrode increases. Many touch sensors are configured such that the external grounded object is a finger. The human body is essentially a capacitor to ground, typically with a capacitance of around 100 pF.
Electrodes in self-capacitance touchpads are typically arranged in rows and columns. By scanning first rows and then columns the locations of individual disturbances induced by the presence of a finger, for example, can be determined. To effect accurate multi-touch measurements in a touchpad, however, it may be required that several finger touches be measured simultaneously. In such a case, row and column techniques for self-capacitance measurement can lead to inconclusive results. As a result, some prior art touchpad sensing systems suffer from a fundamental ambiguity respecting the actual positions of multiple objects placed simultaneously on or near the touchscreen.
One method of overcoming the foregoing problems in self-capacitance systems is to provide a system that does not employ a row and column scanning scheme, and that is instead configured to measure each touchpad electrode individually. Such a system is described in U.S. Patent Publication No. 2006/097991 to Hotelling et al. entitled “Multipoint touchscreen” dated May 11, 2006. In the touchpad sensing system disclosed in the foregoing patent publication to Hotelling, each electrode is connected to a pin of an integrated circuit (“IC”), either directly to a sense IC or via a multiplexer. As will become clear to those skilled in the art, however, individually wiring electrodes in such a system can add considerable cost to a self-capacitance system. For example, in an n×n grid of electrodes, the number of IC pins required is n2. (The APPLE™ IPOD™ employs a similar capacitance measurement system.)
The number of electrodes in a self-capacitance system can be reduced by interleaving electrodes. Interleaving can create a larger region where a finger is sensed by two adjacent electrodes allowing better interpolation, and therefore fewer electrodes. Such patterns can be particularly effective in one dimensional sensors, such as those employed in IPOD click-wheels. See, for example, U.S. Pat. No. 6,879,930 to Sinclair et al. entitled “Capacitance touch slider” dated Apr. 12, 2005.
The second primary capacitive sensing and measurement technology employed in touchpad and touchscreen devices is that of mutual capacitance, where measurements are performed using a crossed grid of electrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheide entitled “Methods and Apparatus for Data Input” dated Jan. 19, 1999 and above-referenced U.S. Patent Publication No. 2006/097991 to Hotelling et al. In mutual capacitance measurement, capacitance is measured between two conductors, as opposed to a self-capacitance measurement in which the capacitance of a single conductor is measured, and which may be affected by other objects in proximity thereto.
In some mutual capacitance measurement systems, an array of sense electrodes is disposed on a first side of a substrate and an array of drive electrodes is disposed on a second side of the substrate that opposes the first side, a column or row of electrodes in the drive electrode array is driven to a particular voltage, the mutual capacitance to a single row (or column) of the sense electrode array is measured, and the capacitance at a single row-column intersection is determined. By scanning all the rows and columns a map of capacitance measurements may be created for all the nodes in the grid. When a user's finger approaches a given grid point, some of the electric field lines emanating from or near the grid point are deflected, thereby typically decreasing the mutual capacitance of the two electrodes at the grid point. Because each measurement probes only a single grid intersection point, no measurement ambiguities arise with multiple touches as in the case of some self-capacitance systems. Moreover, to measure a grid of n×n intersections, only 2 n pins on an IC are needed in such a system.
Some solutions to the problems outlined above are provided by the devices and methods disclosed in U.S. patent application Ser. No. 12/183,456 to Harley entitled “Capacitive Touchscreen or Touchpad for Finger or Stylus” filed Jul. 31, 2008 (hereafter “the '456 patent application”). The devices and methods disclosed in the '456 patent application require, however, a panel or layer that may be deflected downwardly by a stylus into closer proximity with an underlying electrode array. Unfortunately, deflectable panels suitable for this purpose can result in an increased lack of reliability owing to indium tin oxide (“ITO”) traces employed in such devices being thin and brittle, and consequently susceptible to breaking through repeated flexing.
What is needed is a finger touch and stylus capacitive touchscreen that features the advantages of mutual capacitance technology but avoids the disadvantages of self-capacitance technology and deflectable panels. What is also needed is a capacitive touchscreen or touchpad that has the zero-force finger multi-touch navigation capabilities of a traditional capacitive touchscreen in combination with stylus character and text entry and navigation capabilities similar to those provided by resistive touchscreens. What is further needed is a capacitive finger and stylus touchscreen or touchpad that does not absorb or otherwise excessively impede the transmission of light therethrough, and that has a smaller footprint, volume or thickness.
Further details concerning various aspects of some prior art devices and methods are set forth in: (1) U.S. Pat. No. 4,550,221 to Mabusth entitled “Touch Sensitive Control Device” dated Oct. 29, 1985; (2) U.S. Pat. No. 4,686,332 to Greanias entitled “Combined Finger Touch and Stylus Detection System for Use on the Viewing Surface of a Visual Display Device” dated Aug. 11, 1987; (3) U.S. Pat. No. 5,305,017 to Gerpheide entitled “Methods and Apparatus for Data Input” dated Apr. 19, 1994; (4) U.S. Pat. No. 5,670,755 to Kwon entitled “Information Input Apparatus Having Functions of Both Touch Panel and Digitizer, and Driving Method Therefor” dated Sep. 23, 1997; (5) U.S. Pat. No. 5,844,506 to Binstead entitled “Multiple Input Proximity Detector and Touchpad System” dated Dec. 1, 1998; (6) U.S. Pat. No. 6,002,389 to Kasser entitled “Touch and Pressure Sensing Method and Apparatus” dated Dec. 14, 1999; (7) U.S. Pat. No. 6,097,991 to Hamel et al. entitled “Automatic Identification of Audio Bezel” dated Aug. 1, 2000; (8) U.S. Pat. No. 6,879,930 to Sinclair et al. entitled “Capacitance Touch Sensor” dated Apr. 12, 2005; (9) U.S. Pat. No. 7,202,859 to Speck et al. entitled “Capacitive Sensing Pattern” dated Apr. 10, 2007; (10) U.S. Pat. No. 7,436,393 to Hong et al. entitled “Touch Panel for Display Device” dated Oct. 14, 2008; (11) U.S. Patent Publication No. 2006/0097991 A1 to Hotelling et al. entitled “Multipoint Touchscreen” dated May 11, 2006; (12) U.S. Patent Publication No. 2008/0042985 to Katsuhito et al. entitled “Information Processing Apparatus, Operation Input Method, and Sensing Device” dated Feb. 21, 2008; (13) U.S. Patent Publication No. 2008/0055256 to Kwong et al. entitled “Touch Screen Controller with Embedded Overlay” dated Mar. 6, 2008, and (14) U.S. Patent Publication No. 2008/0246496 to Hristov et al. entitled “Two-Dimensional Position Sensor” dated Oct. 9, 2008. Each of the patents and patent applications described hereinabove is hereby incorporated by reference herein, each in its respective entirety.